The invention relates to a glass composition for use in a lamp envelope, which composition comprises silicon oxide, aluminum oxide, barium oxide and at least two alkali-metal oxides.
The invention also relates to a fluorescent lamp comprising a hermetically sealed, glass lamp envelope which is manufactured from such a glass composition.
The lamp envelope of fluorescent lamps is generally made of a cheap, soda-lime type of glass. The lamp envelope may be in the form of a bulb or a tube. A tubular lamp envelope may be straight or, for example, bent so as to be U-shaped or spiral-shaped. The lamp envelope may also comprise a number of parallel, straight tubes which are connected to each other by a so-called bridge. The lamp envelope also includes two so-called glass stems which enable passage of the current-supply conductors. The presence of sodium in the glass has a favorable effect on the shaping and processing of the glass as it reduces the viscosity of the glass. However, it also leads to a number of disadvantages of the lamp.
One of the disadvantages is that the maintenance of the fluorescent lamp is adversely affected by the presence of sodium in the glass. The term maintenance is to be understood to mean in this context the decrease in light output, expressed in lumen, of the lamp during operation. In an ideal case, the light output of the lamp after, for example, 5000 burning hours is equal to the light output at 0 burning hours. It is assumed that said decrease can be attributed to the fact that the phosphors (fluorescent powder) are poisoned by sodium in the glass.
Sodium present in the glass also reacts with the mercury vapor with which the lamp envelope is filled, thereby forming an amalgam. This effect is accompanied by the consumption of mercury by the lamp envelope. In addition, this effect is visible because it causes blackening or greying of the lamp envelope, so that the light transmission of such a lamp envelope is adversely affected. To counteract this adverse interaction with sodium in the glass, the inner surface of the lamp envelope may be provided with a protective coating, for example, of Y.sub.2 O.sub.3. However, this requires an additional process step. In addition, during fusion-sealing, the coating disappears at the location of the bridge and at the location of the stems. It is therefore desirable to use glass having a low sodium content for the lamp envelopes of fluorescent lamps.
In U.S. Pat. No. 4,666,871 a description is given of a glass having a low sodium content for use in a fluorescent lamp. The known glass may contain 0-2 mol % (0-1.3 wt. %) of Na.sub.2 O. However, this known glass has a number of shortcomings.
According to the examples, this known glass comprises large quantities (approximately 30 wt. %) of BaO, which is an expensive raw material. In addition, this component leads to an increase of the liquidus temperature (T.sub.liq) and hence to a considerable increase of the crystallization tendency of the glass.
Another important drawback of the known glass is that it is difficult to manufacture glass tubing and lamp envelopes from this glass because it has a narrow working range (W.R.) and its high T.sub.liq. W.R. is to be understood to mean the difference between the working temperature (T.sub.work) and the softening temperature (T.sub.soft) of the glass. To manufacture glass tubing and lamp envelopes a wide W.R. of at least 300.degree. C. is desirable. The W.R. of the known glass is much narrower, as will be explained hereinbelow.
The known glass may comprise up to 5 mol % of PbO. A disadvantage of the use of PbO is its toxicity. In the preparation of lead glass, PbO is released into the atmosphere by atomization and evaporation, which is harmful to the environment and the operators. PbO is also released when lead glass is subjected to a hot working operation, such as bending, shaping and fusion-sealing. Other disadvantages of PbO are the high price of the raw material and the decrease of the light output of fluorescent lamps, which is brought about by evaporation and, subsequently, condensation of PbO on the fluorescent powder during hot working of the lead-containing glass tubing.